The ear directs, amplifies, and converts sound waves into nerve impulses which are interpreted by the brain. The outer ear or pinna directs the sound waves into the auditory canal terminating at the eardrum or tympanic membrane. Attached to the tympanic membrane on the other side is the malleus, the first of three small bones in the middle ear called the ossicles. The other ossicles are the incus and the stapes. When the eardrum moves in response to the sound waves, the malleus articulates with the incus which in turn articulates with the stapes thereby transmitting the mechanical movement of the eardrum through the middle ear. In addition, the ossicles amplify the mechanical movement by their leveraged relationships. The foot plate of the stapes is attached to a covered resilient oval window in the fluid filled inner ear or cochlea. As the oval window moves in response to the mechanical movement of the stapes, pressure waves are transmitted into the liquid filled cochlea which transduces the pressure waves into electrical signals that in turn are sensed by nerves inside the cochlea. In addition to transmitting the mechanical movement, the oval window amplifies the pressure of the wave appearing at the eardrum because its area is smaller than that of the eardrum. In a normal ear, the mechanical advantage of the ossicles, together with the ratio of the oval window area to the eardrum area, provides a 20 times gain in sound pressure delivered to the cochlea.
Two broad categories of hearing loss are conduction loss and sensorineural (nerve) hearing loss. Conduction hearing loss refers to problems in the conduction of sound from the eardrum to the cochlea while sensorineural loss refers to losses due to defects in the cochlea, the cochlear nerves or the auditory centers of the brain. Measures to alleviate hearing deterioration vary depending on the part of the hearing system that is involved. Apparatus has been designed to amplify acoustical energy and apply it either through its normal path or by vibrating some part of the ossicles. Examples of the former have been available for years. Examples of the latter include implanted magnetic materials, coils, and piezo-electric materials in contact with the ossicles. A third type of apparatus stimulates the cochlear nerves electrically but has a significant disadvantage by requiring the user to relearn the significance of the signals received. A detailed survey of these types of devices is given in U.S. Pat. No. 4,850,962 to Schaefer.
For example, U.S. Pat. No. 3,882,285 to Nunley et al. shows an implant taking sound energy from the auditory canal and stimulating the ossicular chain via a direct mechanical link. U.S. Pat. No. 4,606,329 to Hough uses magnetic coupling through the skin to a coil implanted in the skull. This coil feeds signals in turn to a coil embedded near the middle ear cavity which induces mechanical motion in magnetic material attached to some part of the ossicular chain. In U.S. Pat. Nos. 4,850,962 and 4,729,366 to Schaefer, one of the ossicular bones is removed and mechanical vibration of the tympanic membrane is converted, in an implant, to an electrical signal by a transducer mounted proximate to the terminated ossicular chain. The electrical signals are then applied across the interrupted chain to the promontory of the cochlea or through a hole in the oval window or converted into mechanical motion which is transmitted to the stapes. Other devices are shown in U.S. Pat. Nos. 3,870,832 to Fredrickson; 4,052,754 to Homsy; 4,063,048 to Kissiah; 4,357,497 to Hochmair et al.; and 4,696,287 to Hortmann. Publications of background interest are Hough J. et al. "Experiences with implantable hearing devices and a presentation of a new device" Anno Otol Rhinol Laryngol 95: 1986 60-65, Suzuki J. et al. "Middle ear implant for humans." Acta Otolaryngol 1985:99 313-317, and Hough J. et al. "A middle ear implantable hearing device for controlled amplication of sound in the human: a preliminary report" Laryngoscope 97: February 1987, 141-151.
A fourth type of device produces amplified sound waves in the middle ear. In U.S. Pat. No. 3,346,704 to Mahoney, such a device is described in which a sound receiving and amplifying unit is implanted in the mastoid antrum with a "microphone tube" beginning at a point just under the skin inside the ear canal and a "speaker tube" extending from a speaker into the middle ear space. However, Mahoney has a significant disadvantage in that when acoustical energy from the air is applied directly to the oval window instead of being conducted from the eardrum through the ossicles, the sound transfer is very much reduced because of the greater acoustic impedance of the liquid-filled cochlea compared to that of air. The impedance matching effect provided by the mechanical advantage of the ossicles, together with the ratio of the oval window area to the eardrum area, is lost if the sound energy is introduced directly into the middle ear space.